The present invention relates to a method and apparatus for weighing aggregate, and for determining the quantity of water for mixing concrete and the like, more particularly the present invention is directed to a method and apparatus for weighing the amount (weight or volume) of normal or light weight fine aggregates (sand or metallic, inorganic or organic fibers, such as synthetic fibers) or coarse aggregates (gravel, crushed stone, artificial aggregates) which are utilized in the manufacture of building stocks, civil structural members, etc., such as concrete, mortar, grout, wall structures and coating compositions which utilize such hydraulic substances as cement and plaster the present invention is also concerned with a method and apparatus for determining the quantity of water utilized to admix the hydraulic substances and the aggregates for manufacturing the products described above.
In the manufacture of such products by using the hydraulic substances, the latter are admixed with water and the aggregate (which is not used in the case of manufacturing pastes). To this end, it is necessary to weigh the aggregate and to determine the quantity of water. According to the prior art method, however, it has been extremely difficult to accurately and continuously determine the weight of the aggregate because the weight and volume thereof differ depending upon its water content. Generally, these aggregates are natural products, and even when artifical aggregates are used, they are stocked outdoors so that the quantity of water adhering or contained in the natural and artificial aggregates varies greatly depending upon such weather conditions as rain, sun shine, and atmospheric humidity. Moreover, even in the same lump of the aggregate, the quantity of water adhering to or contained in the aggregate varies continuously from the surface portion to the inside of the lump and the manner of said variation varies substantially. Assuming that the shape and composition of the natural aggregate collected in the same place are the same, the quantity of water adhering or contained in the aggregate varies for the reasons described above. Not only the weight but also the volume of the aggregate vary greatly. For example, the apparent volume is caused to vary by the amount of water. For this reason, the weight of the aggregate measured by the conventional weighing method does not show the net weight thereof. Accordingly, the amount of water determined by such erroneous weight of the aggregate is also not correct. Only when the optimum quantity of water is utilized can products having the maximum strength and the highest quality be produced. Especially, when concrete or mortar is poured into a prepacked mold under a reduced pressure condition according to an invention formally developed by us, the pouring characteristics vary delicately depending upon the quantity of water incorporated within the aggregate which greatly influences the structure and surface condition of the products.
Of course, the fact that the accurate weighing of the aggregate is difficult due to the variation in the quantity of water adhering to or contained in the aggregate has been well known in the art and various efforts have been made to overcome this difficulty. One of the improved methods is to weigh the aggregate under dry state. However, to dry the aggregate it requires a long time to heat and dry it. Such a expedient is possible for treating only a small quantity of aggregate in laboratries but not practical in factories and fields where a large quantity of the aggregate is used. Another method is known as the Inundator method in which the aggregate is weighed while being immersed in water. This method is specified in Japanese Industrial Standard (JIS) A 1109, 1110, 1111, 1134 and 1135. According to this method it is possible to weigh the aggregate in a short time by merely immersing the aggregate in water without the necessity of heating the same for a long time in order to obtain an absolutely dry state. According to this method, however, the following disadvantages appear after the measurement. The immersed aggregate contains a large quantity of water after drainage. Even in the case of a coarse aggregate, the remaining water is such that it is necessary to wipe each aggregate with cloth as prescribed by JIS. In the case of a fine aggregate such as sand, it is extremely troublesome to remove the remaining water. In addition, as the method of measuring the weight and volume of the aggregate in water utilizes the volume of the aggregate and the difference in the specific gravity of water and aggregate the presence of air in and about the aggregate results in a large error in the resulting measurement. For this reason, according to the provision of JIS, the measurement should be performed after completely removing air bubbles from the aggregate by immersing it in water for a long period of time of about 24 hours. In the field, however, immersion in water for such long time greatly delays the job. Immersion of the aggregate in water for 24 hours is too long for modern methods of preparing concrete products according to which the products completely cure and can be taken out from the mold in only several hours. Moreover, in recent years the water to cement ratio has been decreased substantially. For these reasons the Inundator method does not find practical use and accordingly it has been required to intermittently measure the water content of the aggregate. Strict control can be made only by frequent sampling and it has been impossible to accurately determine the water quantity of the entire amount of the aggregate, thus failing to assure the production of products having uniform quality due to uneven fluidity and mechanical strength.